Output signals from amplifiers generally include desired electrical signals and undesired electrical signals that negatively impact device performance. Examples of undesired electrical signals include noise generated internal and/or external to an amplifier and distortions that occur based on the amplifier's non-linear behavior. For instance, many high-speed operational amplifiers utilize bipolar transistors as one of its building blocks to amplify an input signal. However, bipolar transistors typically have characteristics, such as junction capacitances, current gain, collector current, collector-to-emitter voltage, and transconductance that exhibit non-linear transfer functions. As a result, when operating bipolar transistors, such as when driven into saturation or cut-off, the bipolar transistors can exhibit non-linear behavior that causes distortions in an amplifier's output signal.
Distortions that originate from an amplifier's non-linear behavior can include even-order non-linear distortions (e.g., second-order non-linear distortions). As an example, if an amplifier has an exponential transfer function of y=ex, where x is the input signal and y is the output signal, then the exponential transfer function can be expanded into a power series based on equation 1 shown below.ex=1+x+x2/2+x3/6+x4/24+x5/120+ . . . +xn/n!  (1)As an input signal progressively moves farther away from zero and becomes larger (e.g., no longer a relatively small input signal), the second-order distortion term (x2/2), the third-order distortion term (x3/3), and other higher-order distortion terms increasingly affect the output signal and cause non-linear amplification. Often times, the output stage circuit of an amplifier contributes to these even-order nonlinear distortions, in particular, at the low and mid frequency regions. One option in equalizing and reversing these even-order nonlinear distortions is to increase the amplifier's gain bandwidth product (GBP). Unfortunately, increasing the GBP for amplifiers can be power intensive, which can be impractical in low-power environments and designs. As such, being able to equalize nonlinearity distortions without significantly increasing power consumption within amplifiers, such as operational amplifiers, remains valuable in improving device performance.